Surface treatment method and device therefor

ABSTRACT

A surface treatment method wherein one or more active particle streams are generated and aimed at a surface to be treated so that the particle stream interacts therewith. The active particle stream consists of activated particles forming chemically active sites on the surface, and modifying particles occupying said sites. The energy of the activated particles is greater than the energy at break of the inhibited surface bonds of the surface, and lower than the radiative flaw formation energy on the surface. The strength of the particle stream at the treated surface is greater than a quantity N/t where N is the surface density of the inhibited bonds to be broken and t is the duration of exposure of any point on the treated surface to the stream. A device for carrying out the method is also provided.

FIELD OF THE INVENTION

The invention relates to a surface treatment method, in particular forthe surfaces of hard bodies and condensed media. It,likewise relates toa device for implementing the method.

The method and device according to the invention may be used inparticular in the electrical engineering sector, in electronics, thechemical industry, the foodstuffs industry, machine tools, medicine,pharmaceuticals, and in particular for operations involving cleaning,sterilisation, pickling, the deposition of films, surface alloys, etc.

BACKGROUND ART

A treatment method is known for solid surfaces by means of plasma undervacuum, in the course of which a stream of activated particles is formedat low pressure. This method is described in the document “Theiono-plasmic treatment of materials”, Ivanovskii G. F. and Petrov V. I.,Moscow 1986. This particle stream acts on the surface of the solid bodyand the volatile products resulting from the interaction of the plasmaare evacuated from the reaction zone. The composition of the plasma isselected on the basis of the intention behind the treatment. In order toincrease the effectiveness of the operation by plasma under vacuum, thekinetic energy of the particles is increased up to 100 eV and above,which causes the appearance of flaws due to the destructive bombardmentsof the structure of the surface being treated. This treatment, whichrequires the maintaining of the vacuum, is used for preference insituations in which the surfaces to be treated are of restricteddimensions, in particular in the electronics industry.

A dynamic plasma treatment method (Dynamic Plasma Operation or DPO) islikewise known. This method is described in the documents “DynamicalPlasma Operating (DPO) of solid surfaces plasma jets”, Koulik P. P., ed.Solonenko O. P., Fedorchenko A. V., Frunze VSP 1990, pp. 639-653, and“Dynamical Plasma Operating (DPO) of solid surfaces plasma jets”, KoulikP. P. et al., ed. Nouka, 1987, pp. 4-13, 58-96. This consists ofcreating a hydrodynamic stream of activated particles, directing themtowards the surface to be treated, causing them to react with thissurface, and evacuating the volatile products accompanying the reaction,making use of the hydro-dynamic stream pressure; i.e. at high pressure.The surface treated by this method is subject to the action. ofhigh-enthalpy stream, which contain active particles (pickling,cleaning, depositing of films, etc.).

The approximate values of the parameters of the plasma stream are asfollows:

Temperature: 8·10³-15·10³ K

Speed: 100-200 m/s

Density of heat flow on the surface, oriented perpendicular to thestream:

10²⁴-10²⁵ particles/m²·s

For these plasma stream parameter values the interaction of the plasmawith the surface can only be momentary, and does not exceed 10 ms. Thetreated object passes through the plasma stream at a controlled speed(approx. 1 m/s) in such a way that, for transient heat transfer, themaximum surface temperature does not exceed a given limit. According tothe technological operation concerned, this temperature level may beseveral tens of degrees, or even several hundreds of degrees.

Since the duration of the DPO method is less than the characteristicdiffusion time in the solid body, the DPO treatment does not induceflaws in the structure of the solid body treated.

Since the temperature of the electrons on the surface of the body ormedium being treated does not exceed 0.08 eV, and that of the ions,atoms, and other heavy components does not exceed 0.03 eV, it followsthat radiative flaws of the structure of the surface layer treated willbe excluded.

The principal physical energy of the DPO method is expressed by the twoinequalities:

l_(r)<<d<<l_(in)  (1)

where l_(r) is the mean length of the free path of the plasma particles,d is the thickness of the limit layer at the critical point, and l_(in)is the mean diffusion length of the activated particles of the plasma.The left inequality ensures the continuity of the plasma medium, andthat on the right the maintaining of imbalance necessary for aneffective plasmo-chemical action on the surface.

The DPO method is easy to use, in particular at atmospheric pressure.Passage at atmospheric pressure allows for the productivity of thetreatment to be enhanced by 10 to 100 times by comparison with thevacuum plasma process. The quality of the treatment is greater and thetechnology simplified.

Nevertheless, the DPO method can only be used under the followingconditions:

The treatment is applied by a high-enthalpy plasma jet;

Stationary cooling of the objects treated is excluded, and it isnecessary to work under non-stationary heat exchange conditions; i.e. tosubmit the treated surface to a momentary plasma action.

These conditions perceptibly restrict the possibilities of thistechnology, and render the treatment of many materials difficult.

The objective of the invention is to broaden the choice of materialswhich can be treated, to make new surface treatments possible thanks tothe separate monitoring of the activated and stream-modifying functions,and also to broaden the ranges of the physical parameters of the streamsused for the treatment.

BRIEF SUMMARY OF THE INVENTION

To this end, the invention relates to a surface treatment method, inparticular of the surfaces of hard bodies and condensed media, in thecourse of which a stream or streams of activated particles is or arecreated, and directed on to the surface to be treated, and the particlestreams are caused to interact with the surface, and in which thestream(s) of activated particles is (are) composed of activatedparticles, forming chemically active sites on the surface, and modifyingparticles occupying these sites, the energy of the activated particlesbeing greater than the energy at break of the inhibited surface bonds ofthe treated surface, and lower than the radiative flaw formation energyon the surface, the intensity, at the level of the surface treated, ofthe stream of activated particles and the stream of the modifyingparticles being greater than the value N/t, where N is the surfacedensity of the inhibited bonds to be broken, and t is the presence timeof any point on the treated surface, under the stream.

The invention likewise concerns a device for surface treatment for theimplementation of the method, comprising a device for introducing theactive product from an active particle stream generator, including anenergy source, a reactor creating the stream of activated particles, amedium for transporting the activated particles generated to the treatedsurface, and a medium for evacuating the residue products of thetreatment, and in which the active particle generator comprises:

a) Two reactors, the first creating a stream of activated particles,which, when coming in contact with the treated surface, form chemicallyactive sites, the second creating a stream of modifying particles whichhave just occupied the sites;

b) Two transport devices for the activated and modifying particlesrespectively by means of flows carrying the activated and modifyingparticles onto the treated surface, the duration of transport of theactivated and modifying particles from the reactor to the surface to betreated being less than their respective life time durations;

c) A device for the relative displacement of the surface treated inrelation to the streams of activated and modifying particles, ensuringthat the time lapse between the activation and modification actions ofone common area on the treated surface is less than the life time of theactivated sites created by the activated particles, and that the areasof the surface to be treated, being first in contact with the stream ofactivated particles and then only with the stream of modifyingparticles;

d) Two evacuation devices, one for the activated particles deactivatedafter their action on the treated surface, the other for the residualparticles appearing after the action of modification of the surface, thetwo systems being conceived in such a way that the evacuation of the onegroup will not cause any obstacle to the action or evacuation of theothers.

According to one embodiment, the activated particles coincide with themodifying particles, which allows the generator device to be reduced toan introduction device, a reactor, a transport device, a device fordisplacement relative to the surface, and an evacuation device.

According to a variant embodiment, the reactor, the transport andevacuation devices may be common for the activated particles and themodifying particles.

According to another variant embodiment, the evacuation device may becommon for the activated particles and for the residual products.

A description is provided below of the method and device according tothe invention, making reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general diagram illustrating the method;

FIG. 2 is a first embodiment example of the device according to theinvention; the reference figures relating to FIG. 2 are as follows:

1. Argon plasma reactor for the excitation of nitrogen molecules (P=1atm)

2. Plasma pipe and jet containing excited nitrogen molecules (transportof activated particles)

3. Heaters for di-tertiary butyl (H₃ C)₃ C Mg C (CH₃)₃) vapour dilutedin a stream of neutral gas (Ar). Heating temperature: 1000 K

4. Distribution pipe (transport of modifying particles Pg)

5. Polyethylene surface to be treated

6. Transport device providing for the relative movement of the surfaceto be treated in relation to the two reactors. V=5 m/s b=0.1 m Exposuretime for activation 2·10−3 s

7. Evacuation device for the residual activation products

8. Modification ventilation system

9. Energy source.

FIG. 3 is a second embodiment example of the device according to theinvention; the reference figures relating to FIG. 3 are as follows:

1. Reactor for heating and decomposition of the carbon tetrafluoride(T>2000 K), with liberation of F atoms (activated and modifyingparticles coinciding)

2. Argon flow, transporting the F atoms towards the silicon surface

3. Silicon plate

4. Device for moving the silicon plates under the F jet

5. Ventilation (evacuation of residual particles)

6. Energy source

FIG. 4 is a third embodiment example of the device according to theinvention; the reference numbers relating to FIG. 4 are as follows:

1. Reactor, for heating and activating the oxygen and nitrogen moleculesin the air at T approx. 1000 K

2. Pipe forming the jet of excited air and directing it towards theiridium surface

3. Iridium object

4. Device for transporting the iridium object at V approx. 1 m/s

5. Ventilation (device for evacuating the residual products)

6. Energy source

DETAILED DESCRIPTION OF THE INVENTION

In order to resolve the problems of the method of the prior art, thesolution consists of creating one or more streams of particles,directing them towards the treated surface, and causing these particlesto interact with the surface in such a way that, according to theinvention:

The stream of activated particles is formed by particles which, onceactivated by the plasma, form chemically active sites on the surface,

The stream of modifying particles interacts with the surface, in such away as to occupy these active sites,

To do this, the energy of the activated particles is greater than theenergy for breaking the inhibited bonds of the treated surface, andlower than the radiative flaw formation energy,

And the intensity of the stream of activated particles on the treatedsurface, as well as of the stream of modifying particles, is greaterthan the quantity N/t, where N is the surface density of the inhibitedbonds of the treated surface, and t is the presence time of a point ofthe treated surface under the stream.

The essence of the invention rests in the fact that, by contrast withthe DPO method, one or more streams are created which contain speciallychosen particles, one group activated, the other modifying, thefunctions of which are different:

a) The role of the stream of activated particles is to transform thetreated surface into an activated state, i.e. to create chemicallyactive sites thanks to the breaking of the inhibited surface bonds; forexample:

surface bonds which are created as a result of the reconstruction(dimerisation and others) of the surfaces of silicon, gallium arsenic,and other homopolar and heteropolar semi-conductors, of gold, nickel,stainless steel, and other metals and alloys which do not oxidise;

oxide bonds for aluminium and other oxidisable metals,

lateral bonds of radicals in the polymers and bipolymers.

In order for activated particles to be able to play their part, theirenergy Ea must be greater than the energy at break of the inhibitedbond. The energy Ea must, on the other hand, be lower than the radiativeflaw formative energy in order to maintain the quality and structure ofthe surface layer of the treated body.

As the intensity of the stream of activated particles drops, the treatedsurface must be greater than the quantity N/t, where N is the surfacedensity of the inhibited bonds to be broken, and t is the duration ofthe presence of any point of the treated surface beneath the stream. Thequantity N may be lower than the total number of the inhibited bonds ofthe treated surface in the event that the desired treatment does notinvolve the breaking of all the inhibited bonds of the surface.

b) The role of the stream of modifying particles is to occupy thechemically active sites, resulting of the impact of the activatedparticles; i.e. to make use of the activated state of the surfacecreated by the activated particles, in order to operate the desiredtreatment in an effective manner, such as, for example, the depositingof films (with optimum chemical adhesion), pickling (with the formationof volatile, chemically solid molecules), etc.

The processes of formation and bonding of the chemically active sites ofthe treated surface can be developed in parallel. The modifyingparticles can be used in an active state.

The intensity of the stream of modifying particles on the treatedsurface is selected to be greater than the quantity N/t, in such a wayas to make use of all the activated sites created by the activatedparticles. In this case, the treatment is optimum.

In certain particular cases, the activated and modifying particles maycoincide, being of the same chemical nature.

The physico-chemical mechanism of the surface treatment accordinglyconsists of the following:

In the first instance, under the action of the activated particles, theatoms of the surface acquire chemical properties which could have beenobtained, for example, by suddenly removing a surface layer ofmacroscopic thickness of the material which is to be treated. It is thislayer which, under usual conditions, presents an obstacle to theappearance of a chemical activity of the surface.

Accordingly, the effect of the action of the activated particles is tocreate chemically active sites on the surface (radicals).

Next, the modifying particles bond with these sites, which allows forthe desired surface treatment to be carried out in an effective manner.Because the activation state of the surface has a limited life time, theprocess of bonding with these sites must take place within a shortertime than this life time.

This description of the physico-chemical mechanism for the surfacetreatment allows for provision to be made for the possibility (and eventhe necessity) of separate formation of the streams of activatedparticles and modifying particles.

This allows for a check to be conducted separately on the activating andmodifying functions of the stream.

The particles can be activated by different means, not necessarily bythe heating of the gas which they form up to a temperature of (10 to15)·10³ K and the formation of a plasma, a is the case with the DPOmethod. In the present invention, the activation of the particles andthe heating of the gas which they form are processes which areinherently distinct and independent. It is therefore possible to monitorthe heating, i.e. the thermal function of the stream, separately, inaccordance with the demands imposed on the treatment desired.

The particles can be activated, for example by radiation or as a resultof collisions with a stream of charged particles, accelerated in anelectromagnetic field. It is also possible to monitor the streams ofactivated and modifying particles by rapidly reducing or increasing thepressure of the gas which they form.

The range of the physical parameters of the stream acting on the treatedsurface can therefore be very substantially enlarged and enriched.

The density of the stream of particles falling on the treated surfacemust be reasonably great, without the activated sites being able torelax spontaneously, i.e. to revert to their original state before themodifying particles have fixed them. A high density of the flow can beachieved in two ways. The first consists of increasing the speed of theparticles, and therefore their kinetic energy, in the directionperpendicular to the treated surface. This method is however limited tothe formation of the radiative flaws in the structured treated.

The second method consists of increasing the density of the streamparticles. An increase in density leads to the transition of the stream,which passes from the molecular state into a viscous stream. In thiscase, between the falling stream and the treated surface, a limit layeris formed, the thickness of which can be varied in order to vary theintensity of the treatment desired.

The limit layer plays a triple role in the present invention.

1) It plays the part of a barrier for the activated particles which, inorder to reach the treated surface, must be diffused across the limitlayer. The following condition is accordingly imposed on the thicknessof the limit layer:

d<{square root over (6+L D t_(in)+L )}  (2)

where D is the coefficient of diffusion and tin is the life time of theactivated particles. The hydrodynamic qualities of the stream areaccordingly chosen in such a way that the inequality (2) is satisfied.This latter inequality replaces the right-hand inequality in (1).

2) The limit layer is a useful barrier against the return andredepositing on the treated surface of the residual molecules resultingfrom the treatment of the surface (during pickling, for example).

3) The limit layer may play an active role, generating activatedparticles. In this case, the hydrodynamic parameters of the stream areselected in such a way as to satisfy the inequality:

 d₁<{square root over (6+L D t_(in)+L )}  (2)

where d₁ is the distance of the treated surface from the generation areaof the activated particles.

It is evident that d₁<d, a case of which (1) does not take account.

The invention allows for the realisation, apart from known technologieswhich are implemented by the DPO method, of technologies which have notyet been implemented: the adhesive bonding or welding at the molecularlevel of pairs of uniform or heterogenous materials, which cannot bebond or welded to each other by the known methods, the treatment ofnon-regular, particularly natural, organic polycondensates, which areobtained by guiding the activating and modifying thermal functions ofthe stream and broadening the range of physical parameters of the streamused for the treatment.

The present invention can be used in particular for the disinfection andsterilisation of surfaces, for film deposition operations, for thepickling and cleaning of surface alloys in the semi-conductor industry,in order to create bactericidal surfaces, coatings for a variety ofpurposes, etc.

The originality of the process according to the invention rests in theperformance of the following procedures:

1creation of several streams of activated particles,

2application of these streams on the surface to be treated in such a waythat the activated particles of the streams acting on the surface createchemically active sites, and that the modifying particles occupy thesesites,

3this operation is made possible thanks to the provision for theactivated particles of a greater energy than the energy at break of theinhibited bonds of the surface to be treated, and lower than theradiative flaw formation energy, and

4the intensity on the treated surface of the stream of activatedparticles, as well as of the stream of modifying particles is largerthan the quantity N/t, where N is the surface density of the inhibitedbonds to be broken and t is the duration of presence of any point on thesurface beneath the stream.

It will also be noted that the process according to the inventiondiffers from the DPO method due to the fact that:

1the active particle stream is formed of activated particles, creatingchemically active sites on the surface, and the stream of modifyingparticles occupies these sites,

2this is with an energy of the activated particles greater than energyat break of the inhibited bonds of the surface and lower than theradiative flaw formation energy, and

3the intensity on the treated surface of the stream of activatedparticles, as well as of the modifying particles, is greater than thequantity N/t, where N is the surface density of the inhibited bonds atbreak and t is the duration of the presence of any point on the treatedsurface beneath the stream.

The process according to the invention and the device for itsimplementation may be put into effect on equipment of different types.Given that this process allows for highly diverse surface treatments tobe carried out, only the essential elements will be cited which arenecessary for any equipment for the realisation of the invention: Astream generator for the activated particles, a stream generator formodifying particles (the two generators, in certain specific cases, maybe reduced to one single generator), devices for bringing about thecontact between the particles streams and the treated surface, transportdevices which ensure the relative movement of the treated surface andthe activating and modifying flows, devices for the evacuation of theresidual products of the process. The general layout of the process isshown in FIG. 1.

Some embodiments of the devices for the implementation of the processare indicated below, in relation to FIGS. 2 to 4.

EXAMPLE 1 Deposition of Magnesium on Polyethylene

The energy of the inhibited bonds is equal to 4 eV, the surface densityof these bonds is 10¹⁵ cm⁻², and the part of the surface bonds to bebroken is 10%. In view of the thermal instability of polyethylene, theduration of treatment must not exceed 10⁻³ s. The density of the streamof activated particles must therefore not be less than 10¹⁷ cm⁻² s⁻¹.For the quality of activated particles, excited molecules of nitrogen N₂are chosen (B³ Pg, A³S+_(a), aS−_(u), a¹Pg) with activation energyvalues within the limits from 6 to 9 eV. With regard to the quality ofthe modifying particles, magnesium atoms are chosen, obtained thanks tothe thermal decomposition of di-tertiary butyl vapours ((H₃ C)₃ C Mg C(CH₃)₃) diluted in a stream of neutral gas heated to 1000 K. The densityof the stream of modifying particles (magnesium atoms) must be greaterthan 10¹⁷ cm⁻² s⁻¹.

The result is that the polyethylene is covered by a layer of magnesiumwith an adhesion energy of the order of. 4 eV and a partialitycoefficient of 0.1. This device is illustrated in FIG. 2.

EXAMPLE 2 Cold Pickling on Silicon

The energy of the inhibited bonds of silicon is about 1 eV; the totalsurface density of these bonds is about 10¹⁵ cm⁻², and the proportion ofthe surface bonds at break is 100%. To obtain a pickling rate of 10microns/s, the time t of presence beneath the stream for an atomic layershould be about 3·10⁻³ s. Accordingly, the stream of activated particlesmust not be less than 3·10¹⁸ cm⁻² s⁻¹. For the quality of the activatedparticles, fluorine atoms are chosen, thermically obtained thanks to thedecomposition of the carbon tetrafluoride (CF₄) in the stream of gasheated to more than 2000 K. The activation energy in this case is theenergy of the affinity of the electrons to the fluoride atoms, and is3.4 eV. With regard to the modifying particles, fluoride atoms arelikewise used; in this example, the activated and modifying particlescoincide.

The result is that the pickling of silicon is effected at a speed of 10microns/s with energy losses of an order of magnitude lower than that ofthe DPO method. The corresponding device is illustrated in FIG. 3.

EXAMPLE 3 Molecular Oxidation of Iridium

The energy of the inhibited bonds is equal to 0.1 eV, the total surfacedensity of these bonds being 3·10¹⁵ cm⁻², the proportion of the surfacebonds at break being 100%. The time t is of the order of 10⁻² s. Thestream of activated particles must not be lower than 3·10¹⁷ cm⁻² s⁻¹.

For the quality of the activated particles, molecules of oxygen andnitrogen from the heated air (about 1000 K) are chosen. The activationenergy in this case is the thermal energy of the molecules. As modifyingparticles, oxygen molecules are used. The stream of modifying particlescannot be below 3·10¹⁷ cm⁻² s⁻¹ In this example, the real stream of theactivated particles, considering the use of air, far exceeds the limitdemands of the method.

The result is that the surface of the iridium is covered with amono-layer of oxygen molecules, easily decomposable. The energyexpenditure values are two orders of magnitude lower than those of theDPO method.

In this way, the process according to the invention allows for the scopeof utilisation of the DPO method to be broadened, and in particularallows for the creation of new methods of treatment, such as, forexample, the formation of solidifying metallic layers on polymers, whilestill reducing energy expenditure values, thanks in particular to thereduction of the temperature of treatment. The corresponding device isillustrated in FIG. 4.

What is claimed is:
 1. Surface treatment method, for condensed media, inthe course of which at least one stream of active particles is createdand directed at atmospheric pressure onto a surface which is to betreated, and caused to interact with said surface, wherein said activeparticles comprise activated particles, forming at first chemicallyactive sites on said surface, and modifying particles subsequentlyoccupying these sites, the energy of the activated particles beinggreater than the energy of break of inhibited surface bonds of saidsurface and lower than the energy of formation of radiative flaws onsaid surface, the density, at the level of the surface being treated, ofthe stream of activated particles and of the stream of modifyingparticles being greater than the quantity N/t, where N is the surfacedensity of the inhibited bonds to be broken, and t is the presence timeof any point of said surface beneath said at least one stream.
 2. Asurface treatment method as claimed in claim 1, wherein said condensedmedia is a hard body.
 3. A surface treatment method as claimed in claim1, wherein said active particles are selected from excited molecules andexcited atoms.
 4. A surface treatment method according to claim 1,wherein a support device for the surface to be treated is provided, afirst generator creates a stream of said activated particles, saidactivated particles are transported towards the surface to be treatedwith a sufficient speed to allow the formation of chemically activesites on said surface, wherein a chemical precursor compound of saidmodifying particles is introduced into a second generator and saidsecond generator creates a steam of modifying particles, said modifyingparticles are transported onto said surface comprising chemically activesites for occupying said sites.
 5. A surface treatment method accordingto claim 4, wherein said support device for the surface to be treated issubjected to a relative displacement in relation to the streams ofactivated and modifying particles, ensuring that a same zone of thesurface to be treated is at first under the stream of the activatedparticles and subsequently under the stream of the modifying particles,so that the time lapse between actions of activation and modification islower than the life time of the chemically active sited created on thesurface by the activated particles.
 6. A surface treatment methodaccording to claim 4, wherein a particles evacuation means ensuring theevacuation of the activated particles, when totally or partiallydeactivated after their interaction on the surface to be treated, isassociated to the first generator, and in that a particles evacuationmeans ensuring the evacuation of the residual particles resulting fromthe interaction of the modifying particles with the surface comprisingchemical active sited is associated to the second generator.
 7. Asurface treatment method according to claim 1, wherein only onegenerator is ensuring the simultaneous generation of a stream ofactivated particles and a stream of modifying particles.
 8. A surfacetreatment method according to claim 7, wherein a single introductiondevice is ensuring the simultaneous introduction of the particles to beactivated and the chemical compounds precursors of modifying particlesand wherein the generator allows the creation of a single streamcontaining both activated particles and modifying particles.
 9. Asurface treatment method according to claim 1, wherein means fortransporting the activated particles on the one part and the modifyingparticles on the other part, consist in gas streams and in that thespeed of the said streams is such that the transport durations arerespectively lower than the life times of said activated and modifyingparticles.